The present invention relates to an electric power steering device for transmitting the rotation of an electric actuator for generating auxiliary steering power to a vehicle wheel via a worm and a worm wheel.
In light automobiles and small-sized automobiles, there is utilized an electric power steering device, which transmits the rotation of an electric actuator for generating auxiliary steering power to a vehicle wheel via a worm and a worm wheel, and endeavors are being made to lighten the weight and to reduce the noise of this electric power steering device by using a synthetic resin material to form the worm wheel.
Due to the demand in recent years for better fuel economy in order to tackle environmental problems, in an electric power steering device used in a large automobile as well, there has also been a call to form the worm wheel from a synthetic resin material. Thus, because the electric actuator should be made to produce higher output than that of a light automobile, there is a need to improve the dedendum strength, fatigue strength and backlash increase characteristics of the worm wheel, which reduces the speed of rotation of the electric actuator. The backlash increase characteristics correspond to the increment of backlash measured after rotations at a fixed number of times under the condition where the worm wheel meshes with a worm under a fixed load. The excessive backlash causes the noise at the point of contact between the worm wheel and worm to increase.
However, a conventional synthetic resin material worm wheel doesn""t have sufficient dedendum strength, fatigue strength and backlash increase characteristics to be able to withstand a high output electric actuator.
An object of the present invention is to provide an electric power steering device that is capable of solving the above-mentioned problems.
The present invention is an electric power steering device for transmitting the rotation of an electric actuator for generating auxiliary steering power to a vehicle wheel via a worm and a worm wheel, which meshes with this worm, the worm wheel being characterized in that this worm wheel is molded from a synthetic resin material, and, according to a formic acid method, the relative viscosity (VR) of the molded worm wheel is greater than or equal to 100 and less than or equal to 300.
According to the present invention, it is possible to make the electric power steering device lighter weight and less noisy by molding the worm wheel, which transmits the rotation of the electric actuator, from a synthetic resin material. It is possible to improve the dedendum strength of the worm wheel by making the relative viscosity (VR) of the worm wheel molded from the synthetic resin material, measured using the formic acid method, greater than or equal to 100, and also, it is possible to ensure moldability by making the relative viscosity (VR) less than or equal to 300.
The present invention is based on the following knowledge.
In accordance with a general theoretical computation method applied to a gear fabricated from a metallic material, the dedendum strength of a gear is correlated to the tensile strength and the bending strength of the gear material. However, in the case of a worm wheel molded from a synthetic resin material, the dedendum strength is not correlated with the tensile strength and the bending strength of the material. In fact, in a case in which reinforcing fibers were filled into a synthetic resin material in order to improve the tensile strength and the bending strength, the dedendum strength was lower than in a case in which reinforcing fibers were not filled into the synthetic resin material. This is because the dedendum strength of a worm wheel formed from a synthetic resin material is determined in accordance with the balance between the buffering of the surface pressure acting on a tooth of the gear on the basis of the elasticity of the resin material and the strength of the synthetic resin material. The present invention was arrived at by newly discovering that dedendum strength can be improved by increasing the viscosity of the synthetic resin material. That is, in the present invention, the dedendum strength of the worm wheel was increased by making the value of the relative viscosity (VR) of the worm wheel molded from the synthetic resin material 100 or greater, which is larger than in the past, and in accordance therewith, fatigue strength and backlash increase characteristics are improved. Further, in the present invention, moldability can be ensured by making the relative viscosity (VR) of a worm wheel molded from the synthetic resin material 300 or less.
In the present invention, it is desirable that a worm wheel be formed via an injection molding process, and that the gate of the mold for the injection molding thereof is made a film gate. In accordance therewith, even if the relative viscosity of the synthetic resin material injected inside the cavity of the mold is large, it is possible to uniformly fill the inside of this cavity with synthetic resin material, and to prevent moldability degradation.
In the present invention, it is desirable that the synthetic resin material of the worm wheel is a nylon-based synthetic resin material. In a high viscosity nylon, the initial strength of the material itself, such as tensile strength and bending strength, is not different from that of a synthetic resin material of ordinary viscosity, but in a worm wheel molded from a high viscosity nylon, the dedendum strength increases, and performance does not drop in relation to the absorption of moisture and heat degradation even when viscosity increases. Further, a nylon-based synthetic resin material has excellent moldability, and, in addition, can be made long-life even when the relative viscosity is high.
In the present invention, it is desirable that the worm wheel is injection molded from a synthetic resin material. Injection molding can reduce molding costs.
In the present invention, it is desirable that the synthetic resin material of the worm wheel is a pure material. In accordance therewith, the wear of the worm, which meshes with this worm wheel, can be prevented.
In the present invention, reinforcing fibers can be filled into the synthetic resin material of the worm wheel. In accordance with filling reinforcing fibers, it is possible to enhance the dimensional stability of the synthetic resin material by preventing dimensional changes due to heat and moisture absorption.
According to the present invention, it is possible to provide an electric power steering device, which improves the strength of the synthetic resin material worm wheel for transmitting the rotation of the electric actuator for generating auxiliary steering power; makes it possible to increase the output of the electric actuator thereof; enables the conservation of space by making the worm wheel smaller; and furthermore, enables longer life by preventing the lowering of performance in relation to moisture absorption and heat degradation.